Production of sodium sulphide



Patented Dec. 7, 1926.

UNITED STATES PATENT OFFICE.

HORACE FREEMAN, OF SHAWINIGAN FALLS, QUEBEC, CANADA, ASSIGNOR OF ONE-HALF TO CANADA CARBIDE COMPANY, LIMITED, 01'' MONTREAL, CANADA.

PRODUCTION OF sulphide.

The principal objects of the invention are the production of a fusedsodium sulphide product of much greater purity than heretofore.obtainable and the provisionof a process, such that electric furnaces ofthe charge resistor type are rendered available and efiicient for thismanufacture.

Prcliminaryto a disclosure of the invention and for the purpose ofassisting in clearly indicating the novelty of the invention, certainobservations may be made regarding existing commercial methods employedin this art and the products resulting therefrom.

The generally employed method of making sodium sulphide is to reducesodium sulphate by fusing the same, at approximately the temperature ofred heat in a fuel fired furnace, with a carbonaceous reducing agentwhile excluding air. Reduction of the sulphate at first takes placenormally and sodium sulphide is produced, the charge being molten, sothat good contact is obtained between the sulphate and the reducingagent. As reduction proceeds the charge thickens and, when the sodiumsulphide content reaches about 65%, the charge solidifies quitesuddenly. This solidification cannot be prevented'and a charge thussolidified cannot be again fused, even by subjection to the most intenseheat of an electric furnace. Attempts to fuse the charge cause onlyexcessive volatilization of the sodium sulphide. The solidified productis a hot cokey mass. diflicult to handle, which burns rapidly onexposure to the air.

It has been discovered that this phenomenon-of charge solidificationisdue to the infusibility of substantially pure sodium sulphide. In theearly stages of the process, the small -present is uxed by the'largeamount of sodium suI hate, which accounts for the molten con ition ofthe charge in these early stages.

roportion of sodium sulphide SODIUM SULPHIDE.

2, 1925. Serial No. 36,747.

To avoid solidification of the charge in the furnace, the charge isusually drawn when the sulphide content is about 60%. The product,therefore, carries 40% of impurities, being mostly unreduced sodiumsulphate and unu ed reducing agent with perhaps some silica from therefractory lining of the furnace and, if the temperature of operationhas been below 1050 (3., some sodium carbonate. Reasonably pure sodiumsulphide can be obtained from this fused product only by lixiviation andcrystallization. r

Heretofore, attempts have been made to utilize the electric furnace forthis manufacture, because of the high temperatures which could beobtained and which it was thought would permit of the reduction beingcarried to or much more nearly to completion. Such attempts showedpractically no better results than were obtained with fuel firedfurnaces and have not led to commercial operations because of the samedifficulty as experienced in the existing process, namely,solidification when charge was approximately 65% reduced. That is tosay, even in the electric furnace and notwithstanding the application ofhigh electric pressures, the solid fied charge will not carry thecurrent and will not again melt, so that reduction cannot proceedlunlessother means are taken to render the charge fluid and electricallyconductive.

I have found that the disadvantages of the method just described may beovercome and electric furnaces rendered commercially available if thereis added to the furnace charge another substance which will serve toflux the sodium sulphide as it is formed. By the addit on of suitableamounts of such fluxing agents, the charge is maintained freely fluidand satisfactorily electrically conductive throughout the entirereduction period. The freely mobile liquid state of the molten chargesecures perfect contact between the sulphate and the reducing agent, sothat complete reduction may be realized with little or no more than thetheoretical I amount of reducing agent, thus practically eliminating oneof the impurities heretoforeblocked by a solidified charge, as happensnot infrequently with the previous methods. The completely fluid stateof the charge throughout the reduction period produces stable electricalconditions in the furnace,

such that electric furnaces may be economically used to obtain completereduction, which is not commercially practicable in a fuel firedfurnace, even using fluxing agents, because of the great heat absorptionduring reduction.

I have found that a great number of substances possess the property offluxing sodium sulphide, but some of these are of little commercialvalue on account of the relatively large amounts required and becausethey would'constitute undesirably large proportions of impurity in theproduct. I therefore do not contemplate, as embraced in this invention,the use of any substance the amount of which, to maintain free fluidityof the charge at temperatures between 1000 and 1500 C., would be 20%ormore of the product. Generally, I prefer to use only those substanceswhich will represent less than 10% impurity in the product and whichwill maintain the charge liquid at temperatures between 1000 and 1200 C.Certain other substances which have flux-s ing action are undesirablefor various reasons.

As indicating the large number of sub stances available, it may bestated that the following will serve 1. Alkali metal sulphides, otherthan sodium sulphide, such as potassium sulphide, of which less than 5%will sufiice.

2. Carbonates, such as sodium carbonate in large excess or potassiumcarbonate in excess of 20%.

3. Chlorides in excess of 10%.

4. Heavy metal sulphides in amount usually between 5% and 10%.

5. Alkaline earth metal sulphides in certain proportions, such ascalcium sulphide in amount between 20% and 50%.

6. Polysulphides, which are not desirable on account of the fact thatthey are unstable at the temperatures used.

7. Silica in amount between 5% and 10%.

8. Certain sulphates, such as sodium sulphate in amount over 35%.

Dealing, now, with the sulphides which are for many reasons preferred,it may be said that where these can be produced by reduction orotherwise along with the sodium sulphide and without detrimental effect,the corresponding sulphates or any other compounds may be used. Forexample, potassium sulphate may be added to a normal sodium sulphate(salt cake) charge and will be reduced to potassium sulphide.Alternatively, potassium chloride may be added to an acid sulphate(nitre cake) charge and will be reacted on by the acid of I have foundthat less than 5% of potassium sulphide will serve to keep the chargefreely liquid at a temperature of approximately 1000 C. throughouttheperiod of complete reduction of sodium sulphate and, therefore,potassium sulphide is the preferred fluxing agent. Moreover, it is notan undesirable impurity in the product since it functions exactly assodium sulphide in most of the uses of the latter. The potassiumsulphide may be introduced into the charge as such or in theform of anysubstance which will yield it during heating of the charge.

The most desirable temperature for reduction of sodium sulphate tosodium sulphide is between 1000 and 1200 C. At slightly over 1000 C.,say 1050 C., the gaseous product is carbon monoxide and only sul phideis produced, whereas below l000,C. some carbon dioxide is liberated andsodium carbonate is formed. This represents both a loss of sulphide andan undesirable impurity in the product. An additional reason for the useof temperatures over 1000 C. is that less flux is required than attemperatures below 1000 C. y l

The amount of reducing agent added should be sufficient for completereduction of the sodium sulphate according to the equationz with perhapsa slight excess to ensure completion of the reduction within areasonable time and whatever additional amount, if any, may be neededfor production of the fluxing agent. Under these conditions (temperatureagent) and with the use of flux or liquef ing agent, it is possible toproduce fused so ium sulphide of more than purity with and amount ofreducing.

great facility and, by use of the electric,

furnace, at remarkably reasonable cost.

The use of fluxing agents, in addition to enabling complete reduction tobe effected,

enables the electric furnace to be used commercially in thismanufacture; which usev has not heretofore been practicable, and whichin turn is materially instrumental in obtain-- ing complete reduction. Ihave found it best to use the charge as a resistor.

Since all known refractories are fluxed more or less rapidly by sodiumsulphideat the temperatures employed, I have found the most suitabletype of furnace is a steel shell externally water-cooled. The sodiumsulphide, even with the fluxing agent present, has a high melting pointand chills in a thick layer on the cool steeLshell, which layer iselectrically non-conductive and a good thermal insulator. The productmade in a furnace of this type is therefore free from impurities whichotherwise would eniaa ter from the furnace walls and the furnaceinstallation is permanent and inexpensive to maintain.

In carrying out the production of sodium sulphide according to thisinvention, normal sodium sulphate (salt cake) or acid sodium sulphate(nitre cake) with a calculated addition of sodium carbonate toneutralize the acid, is mixed with a properly calcu lated amount of afluxing agent and a suitable carbonaceous reducing agent, such as coalor coke dust, asphalt base oil Or charcoal, having an available carboncontent in amount equal to'or slightly in excess of the theoreticalamount required for reduction of the-sulphate, and the mixture heated toand maintained at a temperature between 1000 and 1500 C. and preferablynot over 1200 C. until complete reduction of the sulphate is eflected,the charge being a mobile liquid during reduction and after reductionuntil tapped from the furnace.

Where reduction or reaction is necessary for production of the fiuxingagent in charge, the necessary reducing agent or reacting substance mustalso be added to the charge.

The product resulting is anhydrous sodium sulphide admixed with thefluxing agent and perhaps a small amount of the reducing agent. Whenusing potassium sulphide as a fluxing agent, the product will containover and usually over available sodium sulphide. Using other fluxingagents, the proportionof impurities in the product will be approximatelyas. indicated in the classification of.these agents.

While certain substances and classes of substances'have been referredto,- it will be understood that this is by. way of explanation andillustration and is not to be construed as limiting the invention to theemployment' of these substances. The term fiuxing agent may be said toinclude and is used in the following claims as including any substancewhich, when added to the charge in amount'not more than 20% of theproduct, will maintain the sodium sulphide in a completely fluid moltenand electrically conductive state at temperatures between 1000 and 1500C., but excluding substances which have detrimental eflect on thereduction process.

Having thus described my invention, what I claim is 1. A process ofmaking sodium sulphide, which comprises reducing sodium sulphate bymelting the same in presence of a carbonaceous reducing agent and afluxing agent capable of maintaining upwards of by melting the same inpresence of a carbonaceous reducing agentand a fluxmg agent capable ofmaintaining upwards of four times its weight of sodium sulphide fluid ata temperature between 1000 and 1200 C.

3. A process of making sodium sulphide, which comprisesreducing sodiumsulphate by melting the same in presence of a carbonaceous reducingagent and a fiuxing agent capable of maintaining upwards of nine timesits weight of sodium sulphide fluid at a temperature between 1000 and1500 C.

4. A process of making sodium sulphide, which comprises reducing sodiumsulphate by melting the same in presence of a carbonaceous reducingagent and a fluxing agent capable of maintaining upwards of nine timesits weightof sodium sulphide fluid at a temperature between '1000 and1200 C. I

5. A process of making sodium sulphide, \vhichcomprises' heating at atemperature between 1000and 1500 C. a mixture initially including sodiumsulphate, a carbona ceous reducing agent and a substance which willunder the operating conditions yield a fiuxing agent capable ofmaintaining upwards of four times its weight of sodium sulphide fluid atthe operating temperature.

6. A process of making sodium sulphide,

-which comprises heating at a temperature between 1000 and 1500 C. amixture initially including a carbonaceous reducing agent, substanceswhich will under the operating conditions yield sodium sulphate, and afluxing agent capable of maintaining upwards of four times'its weight ofsodium sulphide fluid at the operating temperature.

7. A process of making sodium sulphide, which comprises heating at atemperature between 1000 and 1500 C, a mixture in1- tially including acarbonaceous reducing agent, substances which will under the operatingconditions yield sodium sulphate, and potassium sulphide,

8. A process of making sodium sulphide, which comprises heating at atemperature between 1000 and 1500 C. a mixture ini- 3 tially including acarbonaceous reducing agent, sodium sulphate and substances which willunder the operating conditions yield potassium sulphide.

9. A process of maln'ng sodium sulphide, which comprises heating at atemperature over 1000. C..a mixture including a carbonaceous reducingagent, a fluxing agent capable of maintaining upwards of four I timesits weight of sodium sulphide fluid at the operating temperature andsubstances which will under the operating conditions yield sodiumsulphate. 10. An electri furnace process for the production of sod umsul phide, which COII lprises providing in an electric furnace a mixtureof sodium sulphate, a carbonaceous reducing agent and another substancewhich will cause sodium sulphide to melt to fluidity and becomeelectrically conductive, and pass ing through the mixture an electriccurrent sufficient to maintain a temperature above 1000 C. untilreduction of the sulphate is substantially complete, and finally tappingthe resulting molten sulphide from the furnace.

11. An electric furnace process for the production of sodium sulphide,which comprises providing in an electric furnace a mixture of sodiumsulphate, a carbonaceous reducing agent and another substance which willunder the operating conditions yield a lluxing agent which in turn willcause sodium sulphide to melt to fluidity and become electricallyconductive, and passing through the mixture an electric currentsuflicient to maintain a temperature above 1000 C. until reduction ofthe sulphate is substantially complete, and finally tapping theresulting molten sulphide from the furnace.

12. An electric furnace process for the production of sodium sulphide,which comprises providing in an electric furnace a mixture including acarbonaceous reducing agent and substances which will under theoperating conditions yield sodium sulphate and a tluxing agent capableof causing sodium sulphide to melt to fluidity and become electricallyconductive, and passing through the mixture an electric currentsufficient to maintain a temperature above 1000 C. until fluxing agentwhich will under the operating conditions cause sodium sulphide to meltto fluidity and become electrically conductive, and substances whichwill under the operating conditions yield sodium sulphate, passingthrough the mixture an electric current sufficient to maintain atemperature over 1000 C. until reduction of the sulphate issubstantially complete, and finally tapping the resulting moltensulphide from the furnace.

14. An electric furnace process forthe production of sodium sulphide,which comprises providing a mixture including a carbonaceous reducingagent, sodium sulphate and a fluxing agent, in amount not over 20% ofthesodium sulphide obtainable, the said fluxing agent being capable ofmaintaining upwards of four times its weight of sodium sulphide molten,fluid and electrically conductive at temperatures over 1000 (1., andpassing through the mixture an electric current sufiicient to maintainthe same at a temeperature over 1000 C. until reduction is substantiallycomplete.

15. An electric furnace process for the production of sodium sulphide,which comprises providing a mixture including a carbonaceous reducingagent, sodium sulphate and a fiuxing agent in amount not over 10% of thesodium sulphide obtainable, the said fluxing agent being capable ofmaintaining upwards of nine times its weight of sodium sulphide molten,fluid and electrically conductive at temperatures over 100 0., andpassing through the mixture an e1 ctric current sufficient to maintainthe same at a temperature over 1000 C. until reduction is substantiallycomplete.

In witness whereof, I have hereunto set my hand.

HORACE FREEMAN.

